In the United States, marijuana consumption is increasing, while perceived risks of using marijuana are decreasing. This is particularly among young people (Johnston et al., 2014), which suggests that prevalence of chronic and heavy use will increase over time. Scientific literature that should be available to inform public policy, especially harm reduction approaches, has not kept pace with social, political, and legal changes. Marijuana use has been associated with deficits in cognitive function, particularly memory (Solowij & Battisti, 2008). It has also been suggested that regular marijuana use decreases IQ (Meier et al., 2012). Brain-based mechanisms of chronic marijuana use remain poorly understood, and the literature lacks a clear foundation of basic associations between chronic marijuana use and global brain structure. The current proposal seeks to collect structural MRI and cognitive assessments within a mixed sample of older adult chronic marijuana users and age-matched non-users in order to report basic associations between chronic marijuana use and brain structure, as well as correlations between brain structure and cognitive function. It is hypothesized that chronic marijuana users will show global reduced gray and white matter volumes and reduced cortical thickness compared to non-users. It is also hypothesized that chronic marijuana users will show deficits in attention, memory, and processing speed compared to non-users, and that differences in brain structure will be moderately and significantly associated with cognitive function. Findings from the study will increase understanding of the possible risks associated with marijuana use and inform need for harm reduction policies.